Yoghurt production
Yogurt is a fermented milk
product that contains the characteristic bacterial cultures Lactobacillus bulgaricus and Streptococcus thermophilus in a ratio of
1 : 1. Streptococcus thermophilus produces mainly acid
from lactose in the milk causing the pH to drop to about 4-5 from about 7.0. This drop
in pH causes the milk to coagulate. Lactic acid gives yoghurt its sour taste
and limits the growth of spoilage bacteria. Lactobacillus
bulgaricus generates more organoleptic
compounds, particularly acetaldehyde providing the flavour. Proteolytic enzymes and extracellular
polymers from the bacteria also aid protein-gel formation. Yoghurt can be
pasteurized to improve storage-life or remain ‘live’, having probiotic
qualities. Yoghurt is flavored usually with fruits. Full fat yogurt must contain not less than 3.25% milk
fat, low fat yogurt contains not more than 2% milk fat, and nonfat yogurt
contain less than 0.5% milk.
Ingredients
The main ingredient in
yogurt is milk. The type of milk used depends on the type of yogurt – whole
milk for full fat yogurt, lowfat milk for lowfat yogurt, and skim milk for
nonfat yogurt.
Stabilizers such as
alginates (carageenan), gelatins, gums (locust bean, guar), pectins, and starch
may also be used in yogurt to improve the body and texture by increasing
firmness, preventing separation of the whey (syneresis), and helping to keep
the fruit uniformly mixed in the yogurt.
Sweeteners, flavors and
fruit preparations may also be added in yogurt.
Bacterial Cultures
The starter cultures in
yogurt are Lactobacillus bulgaricus
and Streptococcus thermophilus. The
function of the starter cultures is to ferment lactose (milk sugar) to produce
lactic acid. The increase in lactic acid decreases pH and causes the milk to
clot, or form the soft gel that is characteristic of yogurt. The fermentation
of lactose also produces the flavor compounds that are characteristic of
yogurt. Lactobacillus bulgaricus and
Streptococcus thermophilus are the only 2 cultures required to be present
in yogurt. But other bacterial cultures,
such as Lactobacillus acidophilus,
Lactobacillus subsp. casei, and
Bifidobacteria may be added to yogurt as probiotic cultures for improving
lactose digestion, gastrointestinal function, and stimulating the immune
system.
Steps in Yogurt Making
1. Adjust Milk Composition & Blend Ingredients
2. Pasteurize Milk
3. Homogenize
4. Cool Milk
5. Inoculate with Starter Cultures
6. Hold
7. Cool
8. Add Flavors & Fruit
9. Package
1. Adjust Milk Composition
& Blend Ingredients
Milk composition may be
adjusted by adding dry milk powder to achieve the desired fat and solids
content. Ingredients such as stabilizers are also added at this time.
2. Pasteurize Milk
The milk mixture is
pasteurized at 85°C for 30 minutes or at 95°C for 10 minutes. High heat
treatment is used to denature the whey proteins. This allows the proteins to
form a more stable gel and prevents separation of the water during storage. The
high heat treatment also reduces the number of spoilage organisms in the milk.
3. Homogenize
The blend is homogenized to
mix all ingredients thoroughly and improve yogurt consistency.
4. Cool Milk
The milk is cooled to 42°C, the
ideal growth temperature for the starter culture.
5. Inoculate with Starter
Cultures
The starter cultures (Lactobacillus bulgaricus and Streptococcus
thermophilus) are added.
6. Hold
The milk is held at 42°C
until a pH 4.5 is reached. Fermentation results in the formation of a soft gel
and the characteristic flavor of yogurt. This process take several hours.
7. Cool
The yogurt is cooled to 7°C
to stop the fermentation process.
8. Add Fruit & Flavors
Fruit and flavors are added
at different steps depending on the type of yogurt. For set style yogurt the
fruit is added in the bottom of the cup and then the inoculated yogurt is
poured on top and the yogurt is fermented in the cup. For swiss style yogurt
the fruit is blended with the fermented, cooled yogurt prior to packaging.
9. Package
The yogurt is packaged
Health benefits of Yoghurt
Yogurt has been consumed by
humans for hundreds of years, it is highly nutritious. It contains high levels of calcium, Vitamin
B12 and riboflavin, phosphorus, magnesium potassium, Vitamin D, etc. It is a rich source of protein. Yoghurt
contain probiotics that benefit digestive health and immune system.
- Industrial
Microbiology: An Introduction, M J. Waites, N L. Morgan, J S. Rockey, G
Higton
- Modern
Industrial Microbiology and Biotechnology, Nduka Okafor, Science
Publishers
- http://www.milkfacts.info/Milk%20Processing/Yogurt%20Production.htm
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